Electroluminescence devices (EL devices), such as an organic EL device, an LED (light emitting diode), and a semiconductor laser, are structured in such a manner that electrode layers, a light emitting layer and the like are deposited (stacked, superposed or the like) one on another on a substrate. Generally, light generated in the light emitting layer is extracted through a transparent electrode. However, when light enters the interface of the light-extracting-side layer at an angle greater than or equal to a critical angle by influence of the refractive index of each layer, total reflection occurs. Therefore, the light is trapped in the electroluminescence device, and it is impossible to extract the light therefrom. Hence, highly efficient extraction of emitted light is difficult. For example, when the refractive index of the transparent electrode is the refractive index of ITO (indium-tin oxide) or the like, which is often used as the material of the transparent electrode, the light extraction efficiency is said to be approximately 20%.
For example, in an organic EL device, it is known that when an organic material is present in an excited state for a long period of time, the chemical bond of the organic material breaks inherently, and that the light emitting performance of the organic EL device deteriorates as time passes. It is essential to solve this problem when the organic material is used as the material of the electroluminescence device (light emitting device). Further, as long as fluorescence is used, light generation efficiency at an upper level (an upper energy level or state) is theoretically limited to 25%, and it is impossible to increase the light emitting efficiency more than this level. In principle, when phosphorescence is used and intersystem crossing is promoted, it is possible to induce the upper level including only triplets. Therefore, the theoretical limit may be increased to the range of 75% to 100%. However, the lifetime of the triplet in the upper level is longer than that of fluorescence, which is emitted in allowed transition, and the probability of collision between excitons is high. Therefore, the light emitting efficiency is lower. Further, the device deteriorates faster, and the durability of the device is low.
As described above, the extraction efficiency and the light emitting efficiency of the EL device are low. Therefore, the utilization efficiency of the emitted light is extremely low. Hence, the utilization efficiency needs to be improved.
As an approach to improving the light emitting efficiency (or enhancing light emission), W. Li et al., “Emissive Efficiency Enhancement of Alq3 and Prospects for Plasmon-enhanced Organic Electroluminescence”, Proc. of SPIE, Vol. 7032, pp. 703224-1-703224-7, 2008 (Non-Patent Document 1) proposes a method of utilizing a plasmon enhancement effect. In the method of utilizing the plasmon enhancement effect, metal (island form pattern is desirable) is arranged in the vicinity (for example, a few dozens of nanometers) of the light emitting layer to enhance the light emission. Plasmons (or localized plasmons) are induced on the surface of the metal by radiation of dipoles from the light emitting layer. After energy is absorbed, light is radiated again, and the newly emitted light enhances the light emission. Therefore, new transition to light emission by plasmons is added to the light emitting process of the light emitting device. Hence, the lifetime (excitation lifetime) in the upper level can be reduced. When the plasmon enhancement is utilized, improvement in the light emitting efficiency and improvement in durability by reduction of the excitation lifetime may be expected.
However, in Non-Patent Document 1, enhancement of light emission by the plasmon enhancement effect is confirmed only in a light-excitation-type light emitting device (photoluminescence device: PL device), and no successful example has been reported. Non-Patent Document 1 describes that insertion of a metal layer into the EL device causes charge trap and the flow of electrons and positive holes from the electrodes to the light emitting layer are inhibited. Further, Non-Patent Document 1 describes that the carrier balance is broken, and the light emission is suppressed rather than being enhanced.